Physics 152. = N d dt = N. Specifically, Lenz s Law. states: More Faraday. Motional EMF. Announcements. Monday, April 23, 2007

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1 ics Mon pr.23. nnouncements More araday Lenzs Law Motional EM Monday, pril 23, 2007 Help sessions W 9-10 pm in C 119 Masteringics WU #22 due ri., pril 27 Hwk #5 due ri., May 4 o class on Wednesday d t = d Graphically, the slope of the plot of magnetic flux versus time is the negative of the instantaneous induced EM divided by the number of turns,. t t t = t Graphically, the slope of the line connecting the two points in the flux vs time plot is the negative of the average induced EM divided by the number of turns,, over the time interval, t. araday s law tells us the average value of the EM over the time period t in which the magnetic flux is changing. ote the minus sign The induced EM will attempt to oppose the changing magnetic flux. This result is known as = d pecifically, Lenz s Law states: The polarity of the induced EM is such that it produces a whose magnetic field opposes the change in magnetic flux through the loop caused by the external source. tated another way, the induced tries to maintain the original flux through the circuit. 1

2 Worksheet Problem #1 s the bar magnet is brought toward the loop, which way will the flow in through the ammeter? rom Lenz s Law, the will flow through the loop in such a way that the magnetic field it creates opposes the increasing rightward field caused by the approaching bar magnet. o... s the bar magnet is brought toward the loop, which way will the flow in through the ammeter?...the in the circuit will flow counterclockwise as viewed from the bar magnet. t flows away from the left terminal of the ammeter and toward the right terminal. Loops in the presence of changing magnetic fields pick up induced s. The resulting electricity in the circuit can then be used to power appliances (resistors) or charge capacitors. re we getting something for nothing? o, from where is the energy coming? Equivalent Problem otice that as the bar magnet approaches the loop, the induced magnetic field from the loop points opposite the magnetic field of the bar magnet. 2

3 Worksheet Problem #2 Recall that for the case of two bar magnets, like poles repel one another. o WE must do mechanical work to push the bar magnet toward the loop. That mechanical work is what is transformed into the electrical (available energy) the ammeter indicates. nduced due to changing magnetic flux. LEZ LW 100-turn coil with radius 4 cm and resistance 25 is found in a region with a magnetic field perpendicular to the plane of the coil. The magnetic field is decreasing at a rate of 200 T/s. induced? Worksheet Problem #3 B db = 200 T / s = 100 turns R = 25 = (.04) 2 = 5 X 10-3 m 2 x x x x x x x x x v L x x x What happens to the charges in this conducting bar as it moves with a constant velocity v through a uniform magnetic field? The positive charges within the conductor feel a magnetic force upwards. The negative charges feel a downwards force. How long can charge be separated this way? E = B x x x x +++ x x x x x x x x x x x x x x x x x x x x v x x Lx x x x x x x x x x x x x x --- x x x x x x x x --- Until the electrostatic force balances the magnetic force! 3

4 How long can charge be separated this way? E = B E = qe B = qv B x x x x +++ x x x x x x x x x x x x x x x x x x x x v x x Lx x x x x x x x x x x x x x --- x x x x x x x x --- E = vb x x x x x x x x x vx x x L magnitude of the potential difference between the ends of the bar when the electrostatic and magnetic forces are in balance? V = EL = v B L Worksheet Problem #4 Using Ohm s Law: = V / R We saw that the induced EM in a bar of length L is simply: V = B L v ote: You can get the same answer using araday s Law!!! = B L v R x x x xx x x x x x x x x Using araday s Law: E = - d/ = B cos = B cos 0 = B Using araday s Law: E = - d/ 0 d d = d ( B)= db + B d = B d(lx) = BL dx = BLv 4

5 x x x xx x x x x x x x x Worksheet Problem #5 E = d = B L v = B L v R Worksheet Problem #6 5